The present invention relates to a scale inhibitor which is copolymer of sodium vinyl sulfonate and polyalkylene glycol mono- or di-methacrylate, and more particularly, to a process for inhibiting the deposition of scale, particularly inorganic sulfate scale, from aqueous fluids present in or produced from a subterranean formation.
Precipitation of inorganic salts, such as calcium carbonate and calcium, barium, and strontium sulfate, as scale deposits is a persistent and common problem encountered in many field operations for the recovery of hydrocarbons from subterranean formations. Commingling of incompatible aqueous fluids during field operations, especially enhanced oil recovery (EOR) operations involving a waterflood or water drive, results in scale formation and deposition in the formation and in production equipment and tubing. Two or more aqueous fluids are incompatible if each fluid contains distinct ions which form a precipitate and deposit as a scale when the two or more aqueous fluids are commingled. Usually the connate water or brine present in a reservoir will contain barium, calcium and/or strontium ions while water injected into the subterranean formation during EOR operations will contain sulfate ions. For example, offshore operations may involve the injection of large volumes of sea water containing a relatively large concentration of sulfate ions into a subterranean formation having brine containing relatively large concentrations of barium, calcium, and strontium. Upon mixing of the aqueous fluids in situ, precipitation of barium, calcium, or strontium sulfate will occur in the formation and the subsurface and surface production equipment and/or tubing. Commingling of incompatible aqueous fluids usually occurs within the near production well bore environment of a subterranean formation.
Injection of carbon dioxide into a subterranean hydrocarbon-bearing formation as an EOR method results in absorption of carbon dioxide by connate water present in the formation. In addition, some subterranean formation brines, such as those found in the North Sea, may naturally contain a relatively large concentration of carbon dioxide. As pressure is reduced, for example during production, carbon dioxide flashes off to the gas phase thereby increasing the pH of the aqueous fluids and permitting formation of calcium carbonate scale predominantly in the near production well bore environment of the formation and in subsurface and surface production equipment and/or tubing.
Conventional removal of scale which is formed within a subterranean formation and the subsurface and surface production equipment and tubing is costly and ineffective. Scale removal by repeated injection of a chemical agent is relatively expensive. Thus, scale has been removed utilizing various mechanical devices, such as impact jets and/or cavitation jets. As the downtime associated with pulling production tubing and cleaning such tubing above ground is expensive, particularly in offshore locations, wells are cleaned downhole after killing the well. Such mechanical cleaning is time-consuming, relatively inefficient, and potentially hazardous where a radioactive precipitate, for example radium sulfate, is present in the scale to be removed.
Inhibition of scale deposition has been advanced as a more feasible, and accordingly preferable, approach to effectively reducing scale deposition. Conventional commercial scale inhibitors consist primarily of polyelectrolytes, such as polycarboxylates or polyphosphonates. However, the effectiveness of such polyelectrolyte scale inhibitors significantly depends upon the degree of ionization of these inhibitors at the pH value of connate waters. At relatively low pH values, e.g. equal to or less than about 6.0, the effectiveness of a conventional polyelectrolyte scale inhibitor to inhibit barium, calcium, or strontium sulfate scale formation significantly decreases. In addition, conventional polyelectrolyte scale inhibitors utilized to inhibit inorganic sulfate scale dissolve calcium carbonate scale thereby increasing calcium ion concentration which causes undesirable precipitation of the conventional polyelectrolyte scale inhibitors.
Recently, relatively high molecular weight polyvinyl sulfonate has been used to inhibit scale deposition, particularly inorganic sulfate deposition, from aqueous fluid present in and/or produced from a subterranean formation. Such polyvinyl sulfonate is dissolved in an aqueous solution which is preferably squeezed into a subterranean formation. In accordance with a squeeze technique, an aqueous solution of a scale inhibitor, such as polyvinyl sulfonate, is injected into a subterranean formation via a production well bore in fluid communication therewith and can be followed by an overflush, for example a brine containing a relatively low quantity of sulfate ions, i.e. a brine which is compatible with formation fluids. The production well bore may be shut in for a suitable period, for example, zero to twenty-four hours. The polyvinyl sulfonate is absorbed within formation matrix during the shut-in period and is subsequently desorbed over a period of time into aqueous fluids present in and produced from the formation to effectively inhibit scale deposition, particularly inorganic sulfate scale such as barium sulfate scale. After the shut in period, the production well bore is returned to production and aqueous fluids produced from the subterranean formation are analyzed for inhibitor concentration to ensure that an appropriate concentration of inhibitor is present in produced fluids to effectively inhibit scale deposition and to determine the need for subsequent squeeze treatments. Since high molecular weight polyvinyl sulfonates are relatively expensive, a need exists to improve the absorption of such polyvinyl sulfonates into subterranean formation matrix so as to increase the period of time over which polyvinyl sulfonate is subsequently desorbed into aqueous fluids present in and produced from the formation and thereby increase the cost effectiveness of the treatment.
Accordingly, it is an object of the present invention to increase the amount of scale inhibitor which is absorbed within the matrix of a subterranean formation during a process for inhibiting scale deposition.
It is another object of the present invention to increase the period of time over which a scale inhibitor is desorbed from the matrix of a subterranean formation into fluids present in and produced from the subterranean formation in a process for inhibiting scale deposition.
It is a further object of the present invention to increase the cost effectiveness of processes for inhibiting scale deposition in fluids present in and produced from a subterranean formation.